Chemical Recycling of Carbon Dioxide to Chemicals and Fuels

The increasing attention toward climate change is promoting research in the development of suitable strategies for CO2 emissions mitigation. Among these, Carbon Capture and Utilisation strategies (CCUs) appear as the most promising, as they permit to reduce the carbon dioxide emitted in atmosphere, at the same time satisfying the requirements for a sustainable development. Currently, the amount of carbon dioxide consumed in chemical recycling processes represents a very small fraction of the total emissions, thus a net increase in CO2 exploitation through the development of efficient processes and catalysts is required. In this context, the present work deals with the study of different processes aiming to convert CO2 into cyclic carbonates, methane, and methanol, which can cover a very important role in terms of CCUs impact. For each process, suitable heterogeneous catalysts were prepared and extensively characterised by means of different experimental techniques. Cyclic carbonates synthesis through CO2 reaction with epoxides was studied by using imidazolium-based catalysts prepared by means of a one-pot immobilisation procedure over Al_SBA-15 supports having different Si/Al molar ratios. All the catalysts were found highly active in the conversion of both epichlorohydrin and styrene oxide under different reaction conditions. Unlike styrene oxide, epichlorohydrin was found to favour the detachment of the imidazolium moieties more weakly bonded to the surface supports; however, leaching phenomena were observed to occur only during the first catalytic run, highlighting that the prepared materials can be successfully used in multiple catalytic cycles. NiO/CeO2 mixed oxides prepared through two unconventional synthesis procedures were tested for the CO2 hydrogenation to methane under different reaction conditions. A first series was prepared through the “Hard-Template” method, while the second one was obtained by means of the “Soft-Template” strategy. All the prepared materials were found very active and selective toward methanation. The pivotal role of ceria both as promoter and active component was clearly observed; in fact, besides to improve the reducibility and the dispersion of NiO species, ceria is also responsible for the activation of CO2 through the formation of oxygenated intermediates. In addition, the presence of strong Ni-Ce interactions was found to be crucial in ensuring a high catalytic performance also in unfavourable reaction conditions. CO2 hydrogenation to methanol was studied on two different series of Cu-based catalysts, prepared from hydrotalcitic precursors or by the “Soft-Template” approach. The influence of different supports and promoters such as ZnO, Al2O3, ZrO2, and CeO2 on the catalytic performance was investigated. Promising catalytic results were observed by using the catalysts prepared from hydrotalcitic precursors. ZrO2 and CeO2 were found to increase CO2 conversion and CH3OH selectivity, respectively, suggesting that beneficial effects could be obtained by optimizing their concentration. The catalytic results observed by using the Soft-Templated Cu-based samples confirmed the crucial role of ZnO, whose efficient interaction with Cu is needed in order to get high catalytic performances.